The invention relates to centrifugal pumps adapted for use in pumping liquids having a gas content, and more particularly to centrifugal pumps adapted to effect removal of gas from a pumped liquid in order to improve pump performance or processing of the pumped liquid.
It is well known that the presence of a gas, such as air, in a pumped liquid may tend to decrease the hydraulic efficiency of a centrifugal pump, and that gas separated from the pumped liquid may collect in sufficient quantity adjacent the front of the hub or eye of the pump's impeller, as to cause the output of the pump to cease. Separation of gas from the pumped liquid may be due to centrifugal action imparted to the liquid by the pumping vanes of an impeller of the pump or at a point upstream of the impeller adjacent the suction inlet of the pump, such as for instance where it is necessary to employ a separate fluidizer or centrifuge to fluidize or render free flowing a high consistency fibrous pulp suspension for pumping purposes.
It is also known that gas tending to collect in proximity to the hub of the impeller of a centrifugal pump may be removed by providing a flow path defined by flow openings arranged to extend through a shroud or hub of the impeller for purposes of placing front and rear surfaces of the impeller in flow communication; a vent chamber opening through a rear wall of a pumping chamber of the pump adjacent an impeller drive shaft for receiving gas passing through the flow openings and a vent conduit for placing the vent chamber in flow communication with a gas receiving or collecting reservoir, such as the atmosphere, either directly or via an auxiliary vacuum pump, depending on the difference between the suction head, i.e. the pressure existing at the suction inlet of the pump, and the pressure of the gas receiving reservoir. In that there is rarely complete separation of gas from liquid at that point adjacent the front of the impeller with which the flow openings communicate, there is a tendency for a quantity of liquid to escape with the gas through the flow openings, and for this reason it is common practice to provide pump-out vanes on the rear surface of the impeller shroud, which are intended to preferentially act on the liquid component of the gas-liquid mixture passing through the flow openings for purposes of pumping same to the discharge of the pump and thus prevent passage of liquid from the pumping chamber into the vent conduit. Prior pumps of this general type are disclosed for example in U.S. Pat. Nos. 1,101,493; 3,944,406; 4,410,337 and 4,435,193, and Canadian Pat. No. 1,158,570.
It has also been proposed, as in U.S. Pat. No. 3,230,890, to provide an auxiliary pump or centrifugal separator for separating gas from liquid escaping from the pumping chamber of a centrifugal pump.
Under certain steady state conditions, a centrifugal pump fitted with properly sized pump-out vanes may be operated to effect removal of gas in quantities sufficient to permit the pump to operate at an efficiency level corresponding to that characteristic of pump operation with liquid having essentially no gas content without loss of liquid through the vent conduit.
In actual practice, steady state pump inlet conditions are rarely encountered and slight changes in the pressure differential existing across the pump from some predetermined value will either adversely affect the efficiency of a centrifugal pump or allow for loss of liquid through the vent conduit. For example, if the pressure differential across the pump should decrease, due to either a decrease of the suction head and/or an increase in pressure in the gas receiving reservoir, there would be a reduction in the amount of gas removed from the pumped liquid and this would result in a reduction in pump efficiency. There would, however, be no loss of liquid through the vent conduit. Conversely, if the pressure differential across the pump should increase, due either to an increase in suction head or a reduction in pressure in the gas receiving reservoir, pump efficiency would remain essentially the same, but liquid would escape through the vent conduit.
In that in practice it is difficult or impossible to provide a constant suction head and ensure that liquid to be pumped has a uniform gas content, it has been proposed for instance in U.S. Pat. Nos. 3,944,406; 4,410,337 and 4,435,193 and Canadian Pat. No. 1,158,570 to arrange a gas flow control valve in the vent conduit leading to a constant speed vacuum pump and to continuously adjust the valve in a manner determined by sensed changes in various pump operating parameters in an attempt to vary the pressure drop across the pump as required to maximize pump efficiency, while avoiding loss of pumped liquid through the vent conduit.